1. Field of the Invention
The present invention relates to an equalizer and an audio device using the same and, more specifically, relates to an audio device incorporating an equalizer therein such as a graphic equalizer and a surround processor in which the S/N ratio of the incorporated equalizer is improved.
2. Description of Related Art
FIG. 6 is a diagram for explaining a cassette type tape player with an equalizer as an example of conventional audio devices, and shows an outline of an reproducing circuit for one channel. In a stereo type audio device there are provided like circuits for right and left two channels.
Numeral 1 is a read head, 2 a preamplifier, 10 an equalizer, 3 a main amplifier and 4 a speaker.
In these cassette type take players, audio signals magnetically recorded on a tape (not shown) are read via the read head 1, the read audio signals are amplified at the preamplifier 2 and the like, further the sound quality of the audio signals is adjusted by the equalizer 10, the adjusted signals are applied to the main amplifier 3 wherein the signals are power-amplified and are outputted as an audible sound from the speaker 4. Then, a listener can vary the sound quality by manipulating a variable resistor 12 while listening so as to meet his or her tast.
Between a non-inverted input terminal constituting (+) side phase input (hereinbelow simply called as (+) input) and an inverted input terminal constituting (-) side (hereinbelow simply called as (-) input) of an operational amplifier 11 in the equalizer 10, a variable resistor (VR) 12 having resistance R1 is connected, and the movable terminal of the variable resistor 12 is grounded via LCr series resonant circuit 13. The output signal (voltage signal) Vo of the operational amplifier 11 is fed back to the (-) input via a resistor Ro, and an input signal (voltage signal) Vi is applied to the (+) input via a resistor Ri.
Now, when assuming that an open loop gain of the operation amplifier 11 is GA, and further assuming as GA&gt;&gt;I and resistance values of the resistors Ri and Ro as Ri=Ro=R, the following relations stand. Wherein Ri and Ro are respectively resistance values of the resistors Ri and Ro, and the both terminals of the variable resistor 12 are designated as point A and point B as illustrated.
(1) When the movable terminal of the variable resistor 12 is at point A, the input and output relationship at the resonant frequency fo of the series resonant circuit 13 is expressed as follows. Wherein since the (+) input and the (-) input are virtually short-circuited, no current follows through the variable resistor 12, thus the operational amplifier 11 works as a buffer amplifier; ##EQU1## wherein r represents resistance value of the resistor r in the series resonant circuit 13. From the above equation it is understood that the output signal attenuates with respect to the input signal.
(2) When the movable terminal of the variable resistor 12 is at point B, the input and output relationship at the resonant frequency f0 of the series resonant circuit 13 is expressed as follows. Wherein at this moment too no current flows through the variable resistor 12. For this reason the operational amplifier 11 operates as an ordinary amplifier circuit. ##EQU2##
From the above equation it is understood that the output signal is emphasized with respected the input signal.
(3) When the movable terminal of the variable resistor 12 is at the center thereof, the input and output relationship at the resonant frequency fo of the series resonant circuit 13 is expressed as; EQU Vo=Vi
Since Ri=Ro=R, the voltage ratio with respect to the input voltage applied to (+) input and the amplification rate of the operational amplifier 11 are equalized and a same input voltage V1 is generated at (+) input and (-) input.
According to the above equations, it is understood that the circuit explained shows the function of an equalizer having a resonant frequency at the center thereof.
Although in the above explanation a circuit having one series resonant circuit 13 is exemplified, such as CD radio and cassette recorders and component stereo devices comprise a plurality of graphic equalizer circuits each of which covers a predetermined audible subband determined by dividing an audible band into about 5.about.7 subbands. The respective resonant circuits perform a filtering operation for effecting an attenuation or an emphasis on their input signals over a predetermined subband having a predetermined center frequency.
In the circuit of this type, 5.about.7 pieces of such series resonant circuits 13 and variable resistors 12 are respectively connected in parallel between (+) input and (-) input of the operational amplifier 11.
Therefore many number of variable resistors 12 are connected at the input side of the operational amplifier. Since these variable resistors 12 are positioned at the operation panel and are manipulated there, long leading wires thereto are necessitated. Further, since audio signals flow these variable resistors 12, there is a drawback that the equalizer of this type is likely to be affected by noises because of these wirings.
JP-A-63-276312(1988) is exemplified as disclosing a circuit which improves the above drawback. In the improved circuit no variable resistors 12 are provided at the input side of the operational amplifier 11 as illustrated in FIG. 7. Instead, voltage-current converting circuits (hereinbelow simply called as V/I converting circuit) 30 and 31 are provided at the input side of the operational amplifier 11 and the current flowing through these circuits are controlled by their respective variable resistors 12 (not shown). With this measure the variable resistors 12 which adjust the sound quality are isolated from the audio signals and thereby noises are reduced.
The V/I converting circuits 30 and 31 receive input signals via an active filter 32 and convert voltage signals in a predetermined frequency band into current values. The current values converted here are respectively applied to (+) input and (-) input of the operation amplifier 11 through which an attenuated or an emphasized signal with respect to the signals in the predetermined frequency band extracted by the active filter 32 is obtained. Further, the V/I converting circuits 30 and 31 are constituted so as to perform a differential operation each other in response to an operation of a circuit including the variable resistors 12.
With this circuit structure, the noises due to leading wires between the variable resistors and the operational amplifier are reduced and the S/N ratio thereof is improved in comparison with the circuit as illustrated in FIG. 6. However, the present circuit necessitates a resistor Ri connected in series with the input and further the resistance value thereof has to be set substantially the same as that of the feed back resistor Ro in the operational amplifier 11. For this reason the freedom of the circuit designing is restricted as well as the level of the input voltage Vi is also limited. As a result, a sufficient dynamic range can not sometimes be used depending on a bias voltage for an input signal and the S/N ratio is not necessary sufficient.
For overcoming the above problems the applicant proposed a circuit in a JP-A-4-51606 which is hardly limited by a bias for the input signal, can select a large dynamic range and shows a desirable S/N ratio, as illustrated in FIG. 8.
The circuit in FIG. 8 comprises a V/I converting circuit 37 which receives input signals Vi, an active filter 33 which receives the output of the V/I converting circuit 37, an operational amplifier 11, a load resistor RL, another V/I converting circuit 34a and a further V/I converting circuit 35a, and the variable resistor 12 is completely isolated from the V/I converting circuits 34a and 35a via a control circuit 36.
In the circuit, the load resistor RL receives the output of the V/I converting circuit 37. The (+) input of the operational amplifier 11 receives the voltage VL generated at the load resistor RL and the output side of the operation amplifier 11 is fed back via a feed back resistor Ro to the (-) input thereof.
The V/I converting circuit 34a receives the output from the active filter 33, the output side thereof is connected to the (+) input of the operational amplifier 11, in other words the voltage generating side of the load resistor RL and the gain thereof is controlled depending on the current value of a variable current source 34b.
The V/I converting circuit 35a receives the output of the active filter 33, the output thereof is connected to the (-) input of the operational amplifier 11 and the gain thereof is controlled depending on the current value from a variable current source 35b. Further, the current values from the respective variable current sources 34b and 35b for the V/I converting circuit 34a and the V/I converting circuit 35a are controlled by a DC signals from the control circuit 36.
Thus, through provision of the V/I converting circuit 37 receiving the input signal Vi at the input side of the operational amplifier 11 a current corresponding to the voltage level of the input signal Vi is flowed through the load resistor RL to generate a voltage VL. Then the voltage VL is designed to be applied to the operational amplifier 11. Further, in order to attenuate or emphasize signals in predetermined frequency bands among many input signals, the current from the V/I converting circuit 37 is received by the active filter 33 to generate voltage signals corresponding to the predetermined frequency bands and the generated voltage signals are converted by the V/I converting circuits 34a and 35a provided respectively for attenuation and for emphasis. Further, the converted current output from the V/I converting circuit 34a is applied to the load resistor RL to generate a voltage signal and to add here to the input signal Vi. Still further, the output of the V/I converting circuit 35a is applied to the (-) input of the operational amplifier 11 to subtract the same from the input signal. With thus constituting the circuit the insertion of the resistor Ri in series with the input side of the operational amplifier 11 is eliminated.
Since the V/I converting circuits are introduced at the input side of the operational amplifier and a resistor is inserted in parallel with the input of an amplifier circuit such as an operational amplifier, a freedom for setting an input bias is produced as well as a sufficient dynamic range is obtained. Still further, the circuit is not controlled directly by a variable resistor circuit but by a DC signal, a large S/N ratio can be achieved.
The conventional audio device as explained above constitutes such an equalizer in which predetermined frequency bands in the audio signals are extracted via respective filters, each of the extracted frequency bands is divided into two series, one for attenuation and the other for emphasis, and signals of which predetermined frequency bands are attenuated or emphasized are generated and then these attenuated or emphasized signals are added or subtracted to and from the original audio signals. Accordingly, the sound quality thereof is improved in comparison with the other conventional devices, however a further S/N ratio improvement is still required from users using the above audio devices.